G-protein coupled receptors play important roles in diverse signaling processes, including those involved in host defense mechanisms. Immune responses to infectious diseases, injury, tumors and organ transplantation and in diseases and conditions such as asthma, allergy, rheumatoid arthritis and neoplasia have been linked to GPCR regulation. Exaggerated or misdirected immune responses are responsible for many inflammatory and hypersensitivity diseases which, left untreated, can result in tissue or organ damage, pain and/or loss of function. Tissue inflammation is largely implicated in the pathogenesis of such diseases, of which asthma and allergic diseases are among the most well characterized. The mechanisms underlying airway inflammation and hyperreactivity are similar to those underlying allergic inflammation in other tissues, such as the skin and gut.
Prostaglandins are lipid-derived inflammatory mediators that recruit macrophages, T cells, eosinophils, basophils and neutrophils from peripheral blood to damaged or inflamed tissues. In addition, prostaglandins can, depending on the target cell type, induce or inhibit intracellular Ca2+ mobilization, cAMP production, platelet aggregation, leukocyte aggregation, T cell proliferation, lymphocyte migration, and Th2 cell chemotaxis, IL-1a and IL-2 secretion and vascular and non-vascular smooth muscle contraction in responsive cells. Prostaglandins have been implicated in fever, various allergic diseases, vascular and non-vascular smooth muscle relaxation, pain perception, sleep, platelet aggregation and reproductive processes. Prostaglandins exert their effects by interacting with specific GPCRs.
Prostaglandin D2 (PGD2) is the major inflammatory mediator released by activated mast cells, typically found near skin surfaces, mucous membranes and blood vessels, upon immunological challenge (Lewis et al. (1982) J. Immunol. 129:1627-1631). During asthma and allergic responses, PGD2 is released in large amounts. The role of PGD2 in the initiation and maintenance of allergic inflammation has been well established in mouse models of asthma. For example, it has been demonstrated that overproduction of PGD2 in vivo by PGD2 synthase exacerbates airway inflammation in a mouse model of asthma (Fujitani et al. (2002) J. Immunol. 168:443-449).
A PGD2-selective receptor, designated DP, has been identified (Power et al. (1995) J. Biol. Chem. 270:19495-19500). In humans, DP is expressed in smooth muscle, platelets, small intestine and brain, and its expression in lung epithelium is induced by allergic challenge. Receptor activation induces cAMP production and intracellular Ca2+ mobilization, and is believed to inhibit platelet aggregation and cell migration and induce relaxation of various smooth muscles. DP is coupled primarily to G□s protein.
Significantly, in an OVA induced asthma model, DP−/− mice exhibited reduced asthma symptoms, e.g., reduced cellular infiltration of eosinophils and lymphocytes in BAL fluid, reduced Th2 cytokine levels in BAL fluid and reduced airway hyperreactivity to acetylcholine (Matsuoka et al. (2002) Science 287:2013-2019). The increased cellular infiltration in lung tissue and mucus secretion by airway epithelial cells characteristic of asthma in humans and observed in wild-type mice was not observed in DP-deficient mice.
Recently, an additional PGD2-selective receptor, designated chemoattractant receptor-homologous molecule expressed on Th2 cells, or CRTH2, has been identified (Hirai et al. (2001) J. Exp. Med. 193(2):255-261). The receptor was previously referred to as GPR44 or DLIR. Among peripheral blood T lymphocytes, human CRTH2 is selectively expressed on Th2 cells, and is highly expressed on cell types associated with allergic inflammation such as eosinophils, basophils and Th2 cells. It has been shown that CRTH2 activation induces intracellular Ca2+ mobilization and infiltration of Th2 cells, eosinophils and basophils.
Protein sequence analysis indicates that CRTH2 has no significant homology to DP, but rather, is related to members of the N-formyl peptide receptor (FPR) subfamily (Nagata et al. (1999) J. Immunol. 162:1278-1286). In contrast to DP, CRTH2 has been shown to couple primarily to G□i protein.
These observations suggest that CRTH2 and DP may function independently to regulate aspects of allergic inflammation.
The increasing incidence of asthma, allergic diseases and immunologic diseases worldwide underscores the need for new therapies to effectively treat or prevent these diseases. The discovery of small molecules that modulate CRTH2 and/or one or more other PGD2 receptors is useful for the study of physiological processes mediated by CRTH2 and/or one or more other PGD2 receptors and the development of therapeutic agents for asthma, allergic diseases and other immunologic diseases. Novel compounds which display such desirable activity are described herein.
WO 04/058164 discloses certain arylsulfonamide substituted carboxylic acid compounds as asthma and allergic inflammation modulators. From the class of compounds disclosed in WO 04/058164, AMG 009 was selected as the most preferred compound to advance into clinical trials. The structure of AMG 009 is provided below.

When tested in the sheep airway response model, as described in Can J Physiol Pharmacol 1995; 73:191, AMG 009 (1) inhibits antigen-induced late airway response (LAR); (2) blocks antigen-induced development of airway hyper-reactivity (AHR) to carbachol; and (3) blocked allergen-induced recruitment of inflammatory cells to the lung (BAL) (see FIGS. 1, 2 and 3 respectively).
The development of AMG 009 was suspended after unanticipated increases in hepatic ALT/AST levels were observed in healthy volunteers that had received AMG 009. Changes in hepatic function were not anticipated from preclinical safety studies with AMG 009. In vitro metabolism studies revealed that AMG 009 can be metabolically activated to chemically-reactive intermediates capable of forming covalent adducts with proteins. The propensity of AMG 009 metabolism to generate reactive metabolites was conducted in studies to evaluate in vitro covalent binding to protein by standardized methods (Day, et al., J. Pharmacol. Toxicol. Methods., 52, 278-285 (2005)). These studies showed that [14C]AMG 009 radioactive equivalents were bound covalently to protein following incubations with rat and human liver microsomes in the presence of NADPH cofactor at a level of ˜50 pmol equivalent/mg protein. The covalent binding of [14C]AMG 009 to protein in microsomes was in the same range as a target cutoff for acceptable covalent binding in microsomes (50 pmol equivalents/mg protein) as reported in the literature (Evans, et al. Chem. Res. Toxicol., 17, 3-16 (2004)).
The target covalent binding number of 50 pmol equivalents of the drug residue per mg of protein is a target covalent binding value, but is not a threshold. The number of 50 pmol equivalents of the drug residue/mg of protein was not arbitrarily-derived, but came from a thorough literature search of the levels of covalent binding to liver proteins in animals dosed with known hepatotoxins, for example bromobenzene (Monks, T. J. et al., (1982) Life Sci., 30, 841-848), isoniazid (Nelson, S. D. et al, (1978) J. Pharmacol. Exp. Ther., 206, 574-585), and acetaminophen (Matthews, A. M. et al, (1997) Toxicol. Lett., 90, 77-82), under conditions where these drugs induced hepatotoxicity (Evans, D. C. et al, (2004 Chem. Res. Toxicol., 17, 3-16). When the values of covalent binding to protein for these drugs were measured, the levels were as high as 1000 to 2000 pmol equivalents/mg liver protein. Therefore, the covalent binding target adopted by Merck Research Laboratories (Evans, D. C. et al, (2004) Chem. Res. Toxicol., 17, 3-16) is about 20-fold less than that caused by many of these model hepatotoxic drugs.
Many persons of skill in the art currently view chemically-reactive metabolites as an unwanted feature of any drug or drug candidate (Baillie, T. A. (2007) Chem. Res. Toxicol. 2007 Dec. 4 [Epub ahead of print]). Therefore, a goal in drug discovery is to eliminate, or at least to minimize, the metabolic activation liability of drug candidates in that it might assist in leading to the increased probability of safer drugs being successfully developed (Baillie, T. A. et al, (2001) Adv. Exp. Med. Biol., 500, 45-51; Park, B. K., et al (2005) Ann. Rev. Pharmacol. Toxicol., 45, 177-202; Baillie, T. A. (2006) Chem. Res. Toxicol., 19, 889-893; Doss, G. A. and Baillie, T. A. (2006). Drug Metab. Rev., 38, 641-649; Kalgutkar, A. S. and Soglia, J. R. (2005) Expert Opin. Drug Metab. Toxicol., 1, 91-142).
The clinical dose of a pharmaceutical compound is also an important factor, since there have been very few drugs that have been removed from the market for toxicological reasons when the daily dose was less than 10 milligrams (Uetrecht, J. P. (1999) Chem. Res. Toxicol., 12, 387-395).
The compounds of the current invention exhibit unexpectedly improved DP potency, and additionally exhibit improved balance of CRTH2 and DP potencies when compared to the closest compounds disclosed in WO 04/058164, as well as when compared to most preferred compound within that class, AMG 009. This improvement would be expected to allow for a lower clinical dose than that used for AMG 009. Moreover, structural distinctions between the compounds of the current invention and AMG 009 are expected to block metabolism at the metabolic sites found in AMG 009, which may further help to avoid the covalent binding problems that were encountered with AMG 009.